This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which produce extended field of view images.
One form of ultrasonic imaging which has reappeared after a long absence from clinical ultrasonic imaging is the use of compound imaging to create images over an extended length of anatomy. For example, this form of compound imaging, termed panoramic or extended field of view (XFOV) imaging, can be used to produce an image of a blood vessel extending the full length of the leg. But instead of using a single piston transducer to build up such an image line by line, the current techniques use array transducers which produce two dimensional images. The array transducer is moved parallel to the image plane with the anatomy of interest captured in the plane. A succession of two dimensional images are then pieced together to form the XFOV image by aligning the images through two dimensional image correlation, MSAD processing, feature matching, mutual information, and/or other image matching techniques.
One problem which has been recognized in this form of XFOV imaging is the image distortion that occurs due to the scanning direction of each image plane in conjunction with the motion of the scanhead. This distortion can result in a laterally stretched image when the scanhead is moving in the lateral scanning direction, and a laterally compressed image when the direction of scanhead motion and beam scanning oppose each other. While this distortion is not very apparent in an XFOV image, it does become a problem when quantified images are needed, that is, when a measurement is to be made of the anatomical structure in an XFOV image. The distortion can cause a measurement along the length of the image, generally a significant dimension of the anatomy under study in an XFOV image, to be in error by upwards of 10%, depending on scanning speed. A 60 cm measurement can be in error by more than 6 cm.
An approach to correcting this error is to xe2x80x9cwarpxe2x80x9d the image in consideration of these effects back to its correct appearance. A rigorous approach for doing this is found in U.S. Pat. No. 5,910,114. As this patent illustrates, these error sources can be is determined then used to correct every pixel in an image. The correction technique can be applied to a conventional B mode image, or to an elemental image that is going to be used in an XFOV image. However, this intensive correction approach required extensive computation, making the approach difficult and expensive to provide in a real time imaging system. It is preferable to be able to correct this motional error in XFOV imaging without having to resort to expensive or complex computational systems.
In accordance with the principles of the present invention, the spatial lateral error in XFOV images is corrected by estimating the distortional error in an image from knowledge of the scanhead motion and the beam scanning rate and direction. The error is then accounted for when two or more images are aligned to form an XFOV image. Instead of having to perform a complex computation for each pixel, the error is offset in a single step during the image alignment process. While individual segments of the XFOV image may still retain distortion, the size of the image available for measurement has been compensated so that the lateral dimension of the XFOV image is substantially accurate. This correction technique provides quantifiably diagnostic XFOV images without the need for intensive and time-consuming computational processing.